Internal (built-in) functions

PHP comes standard with many functions and constructs. There are also
functions that require specific PHP extensions compiled in, otherwise
fatal "undefined function" errors will appear. For example, to use
image functions such as
imagecreatetruecolor(), PHP must be compiled with
GD support. Or, to use
mysqli_connect(), PHP must be compiled with
MySQLi support. There are many core functions
that are included in every version of PHP, such as the
string and
variable functions. A call
to phpinfo() or
get_loaded_extensions() will show which extensions are
loaded into PHP. Also note that many extensions are enabled by default and
that the PHP manual is split up by extension. See the
configuration,
installation, and individual
extension chapters, for information on how to set up PHP.

Reading and understanding a function's prototype is explained within the
manual section titled how to read a
function definition. It's important to realize what a function
returns or if a function works directly on a passed in value. For example,
str_replace() will return the modified string while
usort() works on the actual passed in variable
itself. Each manual page also has specific information for each
function like information on function parameters, behavior changes,
return values for both success and failure, and availability information.
Knowing these important (yet often subtle) differences is crucial for
writing correct PHP code.

Note:
If the parameters given to a function are not what it expects, such as
passing an array where a string is expected,
the return value of the function is undefined. In this case it will
likely return NULL but this is just a convention, and cannot be relied
upon.